A common problem with conventional radar systems has been the processing of radar return signals to distinguish targets from clutter. A target represents an object that the radar system is intended to detect and about which the radar system provides an indication, such as a "blip" or icon on a radar screen. Clutter represents unwanted indications that are not considered targets.
For the purposes of the present invention, two forms of clutter may produce unwanted indications. Active clutter results from object echoes and jamming signals. Objects whose echoes may cause active clutter include the ground, buildings, the sea, rain, chaff, grass, birds, and the like. Jamming signals are actively and intentionally transmitted in an attempt to trick a radar receiver. On the other hand, passive clutter need not result from any object or active attempt to trick a radar receiver. Rather, passive clutter may result from noise, unintentional interference, and signal processing techniques carried out in a radar receiver.
Conventional radar systems employ hardware receiver designs that generate a range-Doppler or range-range-rate map in which a radar return signal is broken into time components, referred to as range gates herein, and into frequency components, referred to as Doppler bins herein. Each cell in this map characterizes the amplitude of the return signal for the cell's range gate and Doppler bin. Conventional radar system hardware receiver designs limit the amount of passive clutter that appears in this map to acceptable levels. However, the map may include active clutter along with targets. A CFAR processing technique then calculates one or more thresholds based upon the map, and each cell's amplitude is compared to an appropriate threshold times a user-supplied constant. The user may set this constant at a lower level to increase the CFAR or at a higher level to decrease the CFAR. The CFAR processing technique that calculates thresholds exerts a large influence over the radar system results because this algorithm forms the basis for distinguishing targets from clutter, or at least from active clutter.
Unfortunately, the known solutions for minimizing passive clutter in the range-Doppler map involve expensive hardware that often requires periodic error-prone and time consuming alignment and adjustment procedures, complex and heavy equipment, and components that consume an excessive amount of power. While these burdens may be of little importance in some applications, they pose serious obstacles to a portable, battery powered, relatively inexpensive radar unit. Consequently, portable radar units often opt for simple, low power, light weight radar receiver units that place a greater amount of passive clutter in a range-Doppler map.
The conventional CFAR processing techniques are not geared toward distinguishing the spurious signals caused by passive clutter. Consequently, conventional CFAR techniques fail to adequately distinguish targets from both active and passive clutter. In particular, conventional CFAR techniques often fail to identify an unusually high level spurious noise signal, image signal, range sidelobe, or Doppler sidelobe. Thus, conventional portable radar systems too often indicate passive clutter.